1. Field of the Invention
The present invention relates to an infrared optical part for use mainly in infrared spectral analysis, and a method of making the same.
2. Description of the Prior Art
Example uses for infrared optical parts in infrared spectral analysis include use as a window member for infrared spectral analysis, uses as an anvil in organic-material absorption spectrum measurement, and use in infrared ATR prisms.
In infrared spectral analysis, a window member is required for the following purposes, among others: (1) to isolate the detector and/or light source from the external environment, for which purpose the window member is used as, for example, a bolometer window member, a vacuum window member, a dust- and sand-proof window member, or an acid- and alkali-resistant window member; (2) to measure the absorption spectrum or emission spectrum of a sample under special environmental conditions, for which purpose the window member is used as, for example, a cryostat window member, a vacuum-chamber window member, a pressure-cell window member, or a compression window member; and (3) to hold samples in a measuring operation.
Infrared optical materials useful in making such window members include KBr, ZnSe, GaAs, Ge (Miyata, T., "Development of Transparent Optical Parts for High Power CO.sub.2, Laser").
Natural diamond of the IIa type which involves no nitrogen absorption in the infrared region is also used for such window members. However, such diamond available is mostly smaller than 3 mm in diameter and practically little or no supply of such diamond of 5 mm in diameter or more has been obtainable, because the output of a natural IIa type diamond is limited to such a small proportion as 1 to 2%, it being very rare that such diamond is obtained in a relatively large single-crystal size.
The characteristics of the above exemplified conventionally used materials and the problems thereof are shown in Table 1. It can be seen from Table 1 that natural IIa type diamond is characteristically most advantageous; however, the problem is that such diamond is actually unobtainable in any large crystal size.
TABLE 1 __________________________________________________________________________ Characteristic Transmission Refractive index Thermal Material region (.mu.m) at 10.6 .mu.m conductivity W/cmk Problem __________________________________________________________________________ Ge 1.8-13 4.02 0.59 Transmission region narrow CdTe 0.9-13 2.69 0.06 Toxicity and low thermal conductivity GaAs 0.9-18 3.30 0.48 Transmission region narrow ZnSe 0.5-22 2.40 0.18 Transmission region narrow, liable to damage KBr 0.2-30 1.54 0.048 Deliquescent, liable to damage KCl 0.2-24 1.47 0.065 Deliquescent, liable to damage KRS-5 0.5-40 2.38 0.054 Deliquescent, soft, toxic Nat. IIa diamond 0.25- 2.38 20.0 Almost unavailable in large __________________________________________________________________________ size
Hitherto, measurement of the absorption spectrum has been carried out in the following two ways:
(1) A sample is powdered, and mixed and diluted with a material, such as KRS-5, which absorbs no energy in the infrared region, and the same is sintered into a test specimen by hot pressing or otherwise. The test specimen is subjected to measurement by an infrared spectro-analytical apparatus. FIG. 5 is an explanatory view illustrating such method of infrared spectral analysis, in which the test specimen, designated by numeral 41, is placed on a sample holding stage 43 and exposed to a measuring light 42 incident from the light source for spectral measurement by a detector with respect to the transmitted light 44 which has passed through the test specimen. A reflector mirror is shown by the manual 45.
(2) A laminated sample is placed between a pair of opposed diamond anvils processed so as to be able to bear pressure and is crushed the application of pressure into film shape so that it can readily transmit light. The test piece, thus prepared, is subjected to measurement. In this method of measurement, a diamond of the so-called IIa type which has a nitrogen content of 1 ppm or less is selectively used. This way of measurement is schematically illustrated in FIG. 6, in which numeral 51 designates a test piece held between opposed diamond anvils 52, 52, numeral 53 designates an anvil holder, 54 designates a pressing screw, and 55 designates a measuring light beam incident from the light source. The incident light 55 is transmitted by the test piece 51 after being refracted by a reflector mirror 56 and focusing mirrors 57. Again, the light is refracted by another focusing mirror 57 before it is sensed as transmitted light 58 by a detector used for the absorption spectral measurement.
Of these conventional methods, the method described under item (2) is more commonly employed, but this method has the disadvantages as described below.
(1) Since a natural IIa type diamond, the output of which is extremely small, is used, the method is limited in that only a small sized anvil could be used. Therefore, when normal infrared-spectral measuring light (of 2.5-3.0 mm in diameter) is made incident upon a sample, the sample portion, which is of no more than 0.5 mm in diameter, can provide only a limited transmittance of 1/25 to 1/36 of the intensity, it being thus impossible to obtain any proper spectral measurement.
As such, it is necessary to focus the incident light first on the sample portion so that the light which has passed through the sample portion is made into parallel light rays, and then to cause the parallel light rays to be sensed by the detector or to be again focused on the detector, in order to reasonably prevent any loss of incident light intensity. For carrying out this method, a precision-made and expensive optical system is required.
(2) A local portion on which light rays are focused is observed. Unlike any absorption spectrum employed over an entire test specimen, the absorption spectrum thus obtained may provide erroneous information.
(3) Natural IIa type diamond is expensive and its supply is unstable.
(4) It is desirable to use a diamond anvil which is pressure resistant and has a face (100). However, the orientation of the planes of natural IIa type diamond is not clear because it has a curved surface. This makes it difficult to produce a diamond anvil with a face orientation of (100), which is highly resistant to pressure, into exact coincidence with the surface of the sample which is subjected to pressure.
In conventional method of making infrared optical parts, as FIG. 4 shows a, cast disc 1 on the surface of which abrasive grains of diamond have been applied is driven into high speed rotation by a motor 2 and a rotating belt 3. The work 5 to be abraded is set in a fixing jig 4, with a load adjustably applied by weight 6, one end of the jig 4 being placed on table 7; and the work 5 is subjected to abrasion by being pressed against the cast disc 1.
This method involves the following difficulties.
(1) Parallelism cannot be measured during the process of abrading. This makes it necessary to remove the work 5 from the jig 4 to check for parallelism checking and, after parallelism is measured, it is reset in position. In this case, some positional deviation may occur and, as a consequence, the workpiece cannot be set in the desired angular position.
(2) Diamond is so hard that the cast disc 1 is abraded in conjunction with the work. In this case, the cast disc 1 may not uniformly be abraded. Therefore, the desired parallelism cannot be achieved.
(3) Since a large surface area is subject to abrasion, a large load is required. Therefore, a large load is applied to the jig 4, which results in some deformation. As a consequence, the desired parallelism cannot be obtained.